The formation, properties and biological functions of membrane microdomains rich in cholesterol is one of the most exciting and controversial topics in membrane structure and function. In 1994 we proposed (with Deborah Brown, Stony Brook U.) the current working model for the nature of such domains in eukaryotic cells: that they are cholesterol and saturated lipid-rich liquid ordered state lipid domains that segregate from disordered lipid regions rich in unsaturated lipids. A number of pathogenic bacteria, including those in the genus Borrelia, obtain cholesterol from their hosts. In collaboration with Dr. Jorge Benach (Stony Brook U.) the co-discoverer of Borrelia burgdorferi as the causative agent of Lyme disease, we found that B. burgdorferi forms cholesterol glycolipid-containing outer membrane microdomains large enough to directly visualize by electron microscopy. The goal of this project is, in collaboration with the Benach and Brown labs, to understand the physical basis of domain organization of cholesteryl glycolipids and free cholesterol in Borrelia membranes, and to define the functional importance of these domains. Such studies will contribute greatly to our understanding of the general underlying principles of membrane domain organization and function. Electron microscopy, spectroscopic and biochemical approaches will be used to test the hypothesis that B. burgdorferi domains are self-organized ordered lipid domains. To do this, lipid substitution experiments will be used to modify B. burgdorferi membranes in order to determine whether the ability of sterols and other lipids to form ordered domains in vitro is necessary and sufficient for them to form membrane domains in B.burgdorferi cells. Next, whether only those molecules previously shown to strongly associate with ordered membrane domains in model membranes and eukaryotic cells localize within B.burgdorferi membrane domains will be determined. Then, whether B. burgdorferi outer membrane proteins influence domain formation will be determined. To test the hypothesis that interactions specifically involving cholesteryl glycolipids are crucial for domain formation, we will determine which B. burgdorferi lipids have the ability to form membrane domains in model membrane vesicles in vitro. The final aim will be to investigate the function of B. burgdorferi membrane domains. Based on preliminary studies, whether membrane domains are necessary to maintain B.burgdorferi. membrane integrity and/or antibody susceptibility, will be tested. These studies may point the way towards biomedical applications, including prevention/treatment of Lyme disease, by modifying membrane domains. Along these lines, a strategy for combating B. burgdorferi infections by using sterols that are not toxic to eukaryotic cells, but which we have very recently discovered to be toxic to B.burgdorferi cells, will be tested. Ultimately, the studies proposed in this project may lead to a new understanding of the membranes of other cholesterol-containing pathogenic bacteria, and new strategies to combat the diseases that they cause.